Commitments and Contingencies - Litigation (Details) $ in Thousands			1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 28, 2016 USD ($)	Dec. 31, 2006 USD ($)	Mar. 31, 2011 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Mar. 19, 2012 USD ($)
Commitments and Contingencies, Disclosure
Litigation amount as per judgment						$ 16,500
Pretax settlement charge				$ (2,387)	$ 2,327	$ (777)	$ 2,651
Court of Ouargla Foreign Currency Controls
Commitments and Contingencies, Disclosure
Litigation amount as per judgment			$ 25,000
Payment of contract amount in foreign currency		$ 7,500
Payment of contract amount in domestic currency		$ 3,200
Approximate multiplier of the amount at issue for fines and penalties						4
Court of Ouargla Foreign Currency Controls | Maximum
Commitments and Contingencies, Disclosure
Potential judgment in excess of accrual						$ 17,000
NGH2L
Commitments and Contingencies, Disclosure
Pretax settlement charge				$ 16,500		16,500
Escrow Deposit									$ 3,000
NGH2L | ERG Member
Commitments and Contingencies, Disclosure
Payments for settlement	$ 20,000
NGH2L | ERG Member | Parax
Commitments and Contingencies, Disclosure
Monetary damages sought based on alleged breach of contract by NGH2L and alleged tortious interference with contractual relations by Parex	$ 15,000
NGH2L | ERG Member | Maximum
Commitments and Contingencies, Disclosure
Potential recovery								$ 4,500
Monetary damages sought based on alleged breach of contract by NGH2L and alleged tortious interference with contractual relations by Parex						$ 750,000